You can find this in the Mysterious Cave located in the Mysterious Chest in Act II's Oasis area.

In addition, you will need to have Liquid Rainbow to get into 'Diablo 3' Whimsyshire secret level. This item can be bought from Squirt the Peddler in the Caldeum Bazaar in Act II and will cost you 100,000 gold, says MIC. Aside from those, you also need to have Wirt's Cowbell. The easiest way to get this is by going to Quest 9's The Imprisoned Angel and using a waypoint to get to Leoric's Manor from there. The next ingredient that you will need to get into 'Diablo 3' Whimsyshire secret level is the Leoric's Shinbone. Take Leoric's Passage to the Cathedral and check there, says Cheat CC. If you do not find it spawning there and cannot pick one up, you can go to a later quest in Act I and head to the Cathedral Gardens waypoint. You can find it on the ground but choose only the spawning Black Mushroom. To get this you need to seek out the Cathedral in Act I's Level 1 to find one of these. The first ingredient you will need get into 'Diablo 3' Whimsyshire secret level is the Black Mushroom. To get into the 'Diablo 3' Whimsyshire secret level, you will need special ingredients before setting off on your quest. The secret level is easy to get into and all you need to bring with you is persistence. It is a soft world with cupcakes, adorable monsters and other cute trimmings that will make you wonder if you have suddenly left 'Diablo 3' for another game. The level is hidden behind a wall of special ingredients you need to collect. In 'Diablo 3,' there is a secret level of called Whimsyshire.

For those who have not yet explored everything in the game, here is a guide on how to unlock 'Diablo 3' Whimsyshire secret level. 'Diablo 3' fans are elated with the upcoming release of the Necromancer this year.

Diablo 3 liquid rainbow